feat: redesign UI for modern look and clean UX

- Refined dark theme with zinc palette and indigo accents
- Clean sidebar showing only project/session tree
- Moved pattern management to full-screen settings panel
- New session modal with project + pattern picker
- Modern chat with avatar icons, inline send button, Enter-to-send
- Welcome pane when no session is selected
- Added lucide-react for consistent iconography
- Custom scrollbar styling and auto-resize textarea
- Removed clutter: no sidebar pattern list, no verbose headers

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

bun.lock

@@ -6,6 +6,7 @@
       "name": "kopaya",
       "dependencies": {
         "keytar": "^7.9.0",
+        "lucide-react": "^0.577.0",
         "react": "^19.2.4",
         "react-dom": "^19.2.4",
       },
@@ -404,6 +405,8 @@
 
     "lru-cache": ["lru-cache@5.1.1", "", { "dependencies": { "yallist": "^3.0.2" } }, "sha512-KpNARQA3Iwv+jTA0utUVVbrh+Jlrr1Fv0e56GGzAFOXN7dk/FviaDW8LHmK52DlcH4WP2n6gI8vN1aesBFgo9w=="],
 
+    "lucide-react": ["lucide-react@0.577.0", "", { "peerDependencies": { "react": "^16.5.1 || ^17.0.0 || ^18.0.0 || ^19.0.0" } }, "sha512-4LjoFv2eEPwYDPg/CUdBJQSDfPyzXCRrVW1X7jrx/trgxnxkHFjnVZINbzvzxjN70dxychOfg+FTYwBiS3pQ5A=="],
+
     "magic-string": ["magic-string@0.30.21", "", { "dependencies": { "@jridgewell/sourcemap-codec": "^1.5.5" } }, "sha512-vd2F4YUyEXKGcLHoq+TEyCjxueSeHnFxyyjNp80yg0XV4vUhnDer/lvvlqM/arB5bXQN5K2/3oinyCRyx8T2CQ=="],
 
     "matcher": ["matcher@3.0.0", "", { "dependencies": { "escape-string-regexp": "^4.0.0" } }, "sha512-OkeDaAZ/bQCxeFAozM55PKcKU0yJMPGifLwV4Qgjitu+5MoAfSQN4lsLJeXZ1b8w0x+/Emda6MZgXS1jvsapng=="],