Editor: Go to Definition and Go to References are no-ops

[KrisPowers]

F12 (Go to Definition) and Shift+F12 (Go to References) both show up in the editor's right-click context menu and in the Edit menu, but pressing either one does nothing at all. The FullscreenIDE.tsx code acknowledges these are no-ops.

These are arguably the two most important navigation features in a code editor. Without them, understanding unfamiliar code requires manually searching (Ctrl+F) and scrolling.

What needs to happen

There are a few directions to consider:

1. Simple grep-based approach (low effort): When the user presses F12 on a symbol, search the project files for the definition pattern (e.g., function foo, const foo =, class Foo, def foo) and jump to the first result. Not perfect, but far better than nothing.

2. Tree-sitter based approach (medium effort): Since we already parse the file with tree-sitter, we can find the symbol name at the cursor, then search across open files for matching declaration nodes.

3. LSP integration (high effort): Full language server support would make this accurate and complete. This is the correct long-term solution but is a large undertaking.

For a first pass, a regex/grep approach that covers the most common patterns would already be very useful.

Files likely involved

• app/frontend/src/fullscreen/FullscreenIDE.tsx -- command handlers
• app/frontend/src/components/GpuEditor.tsx -- needs a "get symbol at cursor" API
• cpp/src/search.cpp -- could extend the existing content search for definition lookup

Acceptance criteria

• F12 on a function/class/const name navigates to the declaration in the same file or another open file
• Shift+F12 shows a list of locations where the symbol is used
• If nothing is found, show a small "No definition found" notice rather than silently doing nothing