Stacked multi-file diff in all change views, palette browse row, blank-click deselect

[skoonin]

Summary

• Stacked multi-file diff everywhere (closes Show file diffs when a commit is selected in history #5): commit details (History), compare, revision compare, submodule compare, and stashes now follow the same selection-driven model as local changes — no selection shows the whole change set as one scrollable stacked diff, multi-select stacks the selection, single-select shows that file.
• Shared core: the selection dispatch lives once in MultipleDiffContext.BuildDetail; each view supplies only how a change maps to a DiffOption. The working copy keeps its richer dispatcher (staged/unstaged, conflicts).
• Debounced filter rebuilds: typing in a change filter narrows the list live but rebuilds the stacked diff once when typing pauses (~300ms) instead of spawning git diffs per keystroke. Applies to all five filtered views plus the working copy.
• Ctrl/Cmd+P: new "Open Another Repository..." row at the bottom of the repository switcher opens a folder picker for repos not yet registered (offers git-init for non-repo folders).
• Fix: clicking blank space below a file list clears the selection, returning the diff panel to the whole-changeset view.

Review

Multi-angle review (3 correctness + reuse/simplification/efficiency/altitude/conventions finders, verified): no correctness findings; the two accepted findings (per-keystroke rebuild cost, duplicated dispatch) are fixed in this PR.

Testing

• make check green (format, build 0 warnings, 15/15 tests)
• Manual GUI verification of all six views: single/multi/none selection, filter behavior, palette keyboard navigation