Add keyboard shortcuts app-wide, wherever they make sense

[kiranfin]

User Story

As a frequent user, I want keyboard shortcuts for the actions and destinations I use often across the whole app, not just one component, so that I can work without reaching for the mouse.

Context & Motivation

Today the app has only a handful of one-off shortcuts (e.g. Alt+N for a new conversation, plus a few easter eggs), and no discoverable, app-wide set of shortcuts for common navigation and actions. This should cover the app broadly: navigating to frequently used destinations, toggling the sidebar, and other common actions where a shortcut would genuinely save time, not be limited to a single page or component.

Acceptance Criteria

• Frequently used navigation destinations (e.g. Dashboard, Board, Buddy, PM Dashboard, Knowledge Base) have documented keyboard shortcuts.
• A keyboard shortcut toggles the sidebar (collapse/expand, once that feature exists).
• Other frequent, high-value actions get a shortcut where it clearly makes sense (e.g. opening the Buddy, jumping to search), decided case by case rather than exhaustively.
• Shortcuts do not fire while typing in an input, textarea, or other text-entry context, and do not override shortcuts the browser already owns.
• Shortcuts are discoverable: a visible hint on the relevant control and/or a central "keyboard shortcuts" list, not tribal knowledge.
• New shortcuts follow the same conventions as the existing ones (see Technical Notes) so they don't conflict with each other or with the browser/OS.

Technical Notes

There is already a small, deliberate pattern to build on: useNewConversationShortcut.ts implements Alt+N using event.code (physical key, layout-independent) rather than event.key, explicitly avoids Ctrl+N because browsers reserve it, and rejects ctrlKey/metaKey to avoid firing on AltGr chords — reuse this convention for new shortcuts rather than inventing a new one per shortcut. ShortcutHint.tsx is an existing small component that shows a shortcut's keys on hover/focus of the control it belongs to; extend that pattern for discoverability rather than building a separate mechanism.

There is currently no central, app-wide shortcut registry or "keyboard shortcuts" help view — each shortcut so far is a one-off hook wired into a single component. Decide whether this needs a shared registry (to avoid collisions as more shortcuts are added) before spreading many independent keydown listeners across the app.