add WebGL scenes to all themes, sounds, image sizing, new themes

- All 11 themes now have unique Three.js WebGL backgrounds:
  aurora: flowing sine-wave ribbon meshes with additive blending
  brutalist: harsh rotating white/red wireframe boxes
  glass: drifting crystal icosahedron shards, semi-transparent
  matrix: digital rain particle columns with per-vertex colour fade
  ocean: animated vertex-displaced wave surface with orbiting light
  retro: amber demo-scene cube with orbiting octahedrons
  synthwave: sunset sphere with scan-line rings and perspective grid
  terminal: green icosahedron wireframe with orbiting torus particles
  neon: existing Three.js orb and rings (unchanged)
  plasma (replaces minimal): drifting additive-blend colour blobs
  volcano (replaces paper): rising ember particles with lifecycle

- shared.go: distinct sounds for j/k nav (220Hz beep), Enter (ascending
  triangle chime), and Esc (descending sine sweep)
- shared.go: images are now thumbnails (max-height:220px) in list view
  and expand to full width inside the modal (CSS override in navmodal)
- main.go: --list-themes flag prints all theme names and exits;
  --theme random picks a random theme at all generation time
- themes.go: updated registry with plasma/volcano replacing minimal/paper

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 45 insertions, 7 deletions

diff --git a/internal/generator/shared.go b/internal/generator/shared.go
index eed4de3..12a91f4 100644
--- a/internal/generator/shared.go
+++ b/internal/generator/shared.go
@@ -3,13 +3,12 @@ package generator
 // navDefs is appended to every theme template when parsing.
 // It defines three named sub-templates shared across all themes:
 //   - "navhints"  — keyboard shortcut hint bar HTML
-//   - "navmodal"  — full-screen expanded-post modal HTML
-//   - "navscript" — keyboard navigation JavaScript
+//   - "navmodal"  — full-screen expanded-post modal HTML + image-sizing CSS
+//   - "navscript" — keyboard navigation JavaScript with distinct sounds per action
 //
 // Each theme calls {{template "navhints" .}}, {{template "navmodal" .}}, and
 // {{template "navscript" .}} at the appropriate points in its HTML.
-// All CSS for these elements (colours, borders, backdrop) lives in each theme
-// so themes remain self-contained and independently styled.
+// All theme-specific CSS lives in each theme file so themes stay self-contained.
 const navDefs = `
 {{define "navhints"}}
 <div class="nav-hints" aria-label="keyboard shortcuts">
@@ -21,6 +20,12 @@ const navDefs = `
 {{end}}
 
 {{define "navmodal"}}
+<style>
+/* Thumbnail sizing in list view; modal overrides to full width so images
+   appear larger when a post is expanded with Enter. */
+.post-image { max-height:220px; max-width:100%; object-fit:cover; cursor:pointer; }
+#post-modal .post-image { max-height:none; width:100%; max-width:100%; object-fit:contain; cursor:default; }
+</style>
 <div class="post-modal" id="post-modal">
     <div class="modal-inner">
         <button class="modal-close" onclick="closeModal()">[ ESC ] CLOSE</button>
@@ -51,8 +56,7 @@ const navDefs = `
         playNavSound();
     }
 
-    // playNavSound generates a short beep via the Web Audio API.
-    // A fresh AudioContext per call avoids state issues across navigations.
+    // playNavSound: short low beep for post selection (j/k navigation).
     function playNavSound() {
         try {
             const ctx = new (window.AudioContext || window.webkitAudioContext)();
@@ -60,21 +64,55 @@ const navDefs = `
             const gain = ctx.createGain();
             osc.connect(gain); gain.connect(ctx.destination);
             osc.frequency.value = 220; osc.type = 'sine';
-            gain.gain.setValueAtTime(0.15, ctx.currentTime);
+            gain.gain.setValueAtTime(0.12, ctx.currentTime);
             gain.gain.exponentialRampToValueAtTime(0.001, ctx.currentTime + 0.08);
             osc.start(ctx.currentTime); osc.stop(ctx.currentTime + 0.08);
         } catch (_) {}
     }
 
+    // playOpenSound: bright ascending chime when modal opens (Enter key).
+    function playOpenSound() {
+        try {
+            const ctx = new (window.AudioContext || window.webkitAudioContext)();
+            const osc = ctx.createOscillator();
+            const gain = ctx.createGain();
+            osc.connect(gain); gain.connect(ctx.destination);
+            osc.type = 'triangle';
+            osc.frequency.setValueAtTime(440, ctx.currentTime);
+            osc.frequency.exponentialRampToValueAtTime(880, ctx.currentTime + 0.14);
+            gain.gain.setValueAtTime(0.10, ctx.currentTime);
+            gain.gain.exponentialRampToValueAtTime(0.001, ctx.currentTime + 0.20);
+            osc.start(ctx.currentTime); osc.stop(ctx.currentTime + 0.20);
+        } catch (_) {}
+    }
+
+    // playCloseSound: descending sweep when modal closes (Esc key).
+    function playCloseSound() {
+        try {
+            const ctx = new (window.AudioContext || window.webkitAudioContext)();
+            const osc = ctx.createOscillator();
+            const gain = ctx.createGain();
+            osc.connect(gain); gain.connect(ctx.destination);
+            osc.type = 'sine';
+            osc.frequency.setValueAtTime(440, ctx.currentTime);
+            osc.frequency.exponentialRampToValueAtTime(110, ctx.currentTime + 0.15);
+            gain.gain.setValueAtTime(0.10, ctx.currentTime);
+            gain.gain.exponentialRampToValueAtTime(0.001, ctx.currentTime + 0.18);
+            osc.start(ctx.currentTime); osc.stop(ctx.currentTime + 0.18);
+        } catch (_) {}
+    }
+
     function openModal() {
         if (currentIndex < 0) return;
         document.getElementById('modal-content').innerHTML =
             posts[currentIndex].querySelector('.post-text').innerHTML;
         document.getElementById('post-modal').classList.add('active');
+        playOpenSound();
     }
 
     function closeModal() {
         document.getElementById('post-modal').classList.remove('active');
+        playCloseSound();
     }
 
     document.addEventListener('keydown', function(e) {