Block zoom / focus mode with breadcrumb

[avelino]

Summary

Implement "zoom in" on a block to see only that block and its children, hiding everything else. This is one of Roam's most used navigation features for working with deep hierarchies.

Roam web behavior: Clicking a block's bullet zooms into it. The page shows only that block's subtree with a breadcrumb trail at the top. Click any breadcrumb segment to zoom out to that level.

TUI adaptation: Use a dedicated keybinding (e.g., z in normal mode for vim) to zoom into the selected block. Breadcrumb displayed in the header area. A "zoom out" / "back" key (e.g., Backspace or Z) returns to parent level. This replaces mouse-click-on-bullet interaction.

Implementation hints

State

• src/app.rs:70-91 — add zoom context to AppState:

  pub zoom_path: Vec<ZoomLevel>, // stack of { block_uid, block_text_preview }

• When zoom_path is non-empty, rendering should filter to only show the zoomed block's children
• Empty zoom_path = normal full view

Block resolution

• src/app.rs:158 — resolve_block_at_index() maps flat index to block info
• When zoomed, the flatten logic in ui/main_area.rs should start from the zoomed block instead of root
• src/ui/main_area.rs:277-295 — the flatten_blocks() function needs a "root block" parameter

Breadcrumb rendering

• src/ui/header.rs — extend header to show breadcrumb when zoomed:
  Graph: my-graph > Daily Note > Parent Block > Current Block
• Each segment is the first ~20 chars of the block text (truncated)
• In the web UI this is clickable; in TUI we navigate with the zoom-out key

Navigation

• Zoom in: sets zoom_path.push(current_block), re-renders from that subtree
• Zoom out: zoom_path.pop(), returns to parent view
• Multiple zoom levels stack (zoom into child of zoomed block)
• Going "all the way out" clears zoom_path entirely

Keybindings

• src/keys/preset.rs — add new actions: ZoomIn, ZoomOut
• vim: z (zoom in), Z or Backspace (zoom out)
• emacs: Ctrl+] / Ctrl+[
• vscode: Enter (zoom in), Backspace (zoom out)

Acceptance criteria

• Keybinding zooms into selected block showing only its subtree
• Breadcrumb trail in header shows zoom path
• Zoom out returns to parent level
• Multiple zoom levels work (zoom into child of zoomed block)
• Selection resets to first block when zooming in/out
• Edit, create, delete, indent/dedent all work while zoomed