feat(branch-panel): add branch filtering, persistent view state, and UI polish
- Add a text filter with highlight and clear (Esc to clear) that narrows visible branches and auto-opens folders while filtering. - Persist panel view state (local/remote open + collapsed folders) to localStorage under "gitlite.branchPanelView.v2" so folder open/collapse and section visibility survive reloads. - Reveal current branch action: clears filter, expands path to current branch, scrolls it into view and briefly flashes it. - Rework branch list rendering: unified folder/branch ids, scope-aware rows, tracking status computation (tracked/gone/local/remote-tracked), richer titles/tooltips, updated icons, and better context-menu positioning/behavior (separate showBranchMenuAt + open-from-button). - Prevent toggling folder collapse while filtering; folder rows are implicitly open when a filter is active. - Add slim, rounded custom scrollbars for the left sidebar in src/app.css. No external APIs changed; behavior is additive and intended to improve branch navigation and discoverability.
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