Feat/remove selection fab context menu

[Omm2005]

What changed

• Removed browser context menu integration from the extension.
• Removed selection FAB/search overlay behavior.
• Deleted selection-search content entrypoint.
• Removed related message constants and DOM IDs.
• Removed contextMenus permission from extension manifest config.

Why

• Dhravya said lol

Files changed

• apps/browser-extension/entrypoints/background.ts
• apps/browser-extension/entrypoints/content/index.ts
• apps/browser-extension/entrypoints/content/selection-search.ts (deleted)
• apps/browser-extension/utils/constants.ts
• apps/browser-extension/wxt.config.ts

Testing

1. bun install
2. cd apps/browser-extension
3. bun run dev
4. Load extension in dev mode.
5. Select text on a website and confirm no FAB appears.
6. Right-click on page/selection and confirm no supermemory context menu items exist.

Breaking changes

• Removed selection search/save flow from content script.
• Removed extension context menu actions.