Redesign top bar + sidebar, add settings (#5)

[mvbmir]

Summary

Implements the cmux-style top bar and sidebar redesign plus three related settings and the split-ratio fix. Single commit against main.

Top bar

Custom top bar at the top of the window: sidebar toggle, settings cog, "+" new-workspace button, a row of workspace indicator pills, and the window controls (minimize/maximize/close). Empty space drags the window via gtk::WindowHandle. The window controls are plain gtk::Buttons rather than gtk::WindowControls so their hover shape and sizing match the rest of the icons.

Pane header

• Empty space between the last tab and the action icons is a WindowHandle filler — drags the window without conflicting with the tabs' DragSource.
• Middle-click on a tab closes it.
• Settings cog moved out of the per-pane action row and into the top bar, freeing horizontal tab space.

Sidebar

• Each row has a close X on the top-right (replaces the old star slot). Star moved below next to the folder path; both are hover-to-reveal 20×20 buttons.
• Double-click a workspace row to inline-rename.
• Accent-tinted selected-row background; first-row top margin matches the between-workspace gap.
• No more "WORKSPACES" title or big bottom "+" button.

New-workspace UX

• Clicking "+" clones the active workspace's folder. The folder picker only appears on first run / when no workspace exists.

Settings → General (three new rows)

• Top bar (switch) — when off, the top bar is removed entirely. Its buttons relocate into a new sidebar header (sidebar open) or onto the active workspace's leading pane (sidebar collapsed). Sidebar-header layout puts the app buttons on one end and the window controls on the other with an hexpand spacer between.
• Window controls side (Left / Right) — moves minimize/maximize/close between the two ends of whichever header is active.
• Workspace indicators on the top bar (switch) — hides the pills without hiding the top bar. Implementation hides each pill individually so indicator_box keeps its hexpand spacer role.
• All three persist under interface.* in settings.json. Defaults: show_top_bar = true, show_workspace_indicators = true, window_controls_side = Right — upgrading users see the familiar layout.

Split ratio fix

• shrink_start_child / shrink_end_child on every gtk::Paned so the saved ratio wins over larger child minimums (e.g. a wide tab strip).
• position-notify now tracks last_size: width changed → auto-adjust (don't touch ratio); width unchanged → real user drag (update ratio). Fixes the silent 0.5 → 0.6 drift that happened when the sidebar toggled or the window resized — GtkPaned::position is absolute pixels, so the old handler computed a different ratio each time the paned's width changed.
• Per-frame tick callback observes the paned's actual width and re-applies position = ratio × new_width on size changes. GtkWidget::width / height don't reliably emit notify across GTK 4.x versions, so polling is intentional; the check is O(1) with early return.
• Startup uses a one-shot tick callback to apply the ratio once the paned first has a non-zero allocation.

Structural

• handle_config_change() dedupes the appearance + top-bar side-effect + save/revert logic that ran from two config-change sites.
• apply_top_bar_mode() is the single source of truth for how the dock toggle, settings cog, +, indicator pills, and window controls are laid out — respects both the persistent show_top_bar setting and the transient keyboard toggle.
• Widget tree walkers (is_pane_widget, find_leaf_focused_pane) unwrap gtk::WindowHandle so the dock-parked-on-pane layout still resolves panes correctly.

Closes #5 (notifications explicitly out of scope per the issue).

Notes for reviewers

• The add_tick_callback polling in split_tree.rs is intentional. GTK 4.x doesn't reliably emit notify on width/height, so the signal-driven approach silently misses resize events; the tick loop catches them at the cost of one integer compare per frame.
• gtk::WindowControls is replaced by three plain gtk::Buttons wired to window.minimize() / maximize() / close(). Adwaita's default 24 px circular styling couldn't be cleanly overridden to match the other top-bar icons.

Test plan

• cargo fmt clean
• cargo clippy --release -- -D warnings clean
• cargo test — all 181 unit tests pass
• Manual: top bar on/off, sidebar open/closed, left/right controls — all combinations
• Manual: workspace indicators on/off toggles the pills; window controls stay at the right edge regardless
• Manual: middle-click tab close, last tab closes pane, last pane closes workspace
• Manual: split ratio stable across sidebar toggles and window resizes
• Manual: double-click inline rename, folder clone on "+"

[mvbmir]

@am-will Apologies for the big PR, adding a top bar is opinionated design wise, considering you don't have one, but since I really felt the need for it, I added it as an optional UI part.

So, for people who like the no top bar, they can keep the dock only, with slight improvements.

There's also some other quality of life things like closing tabs with middle mouse button, making the pane bars drag the window also, some button designs, etc, so if you see anything you'd like me to separate into a different PR let me know.

[am-will]

I will take a look when I can ty

[mvbmir]

Hi @am-will, did you end up taking a look?

[am-will]

My fault dude I got sick. I'll check later

…

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[am-will]

let me merge some other issues and i'll get to this. looks interesting for sure