Add Vim keybindings mode for SQL editor

[muk2]

Summary

Implements Issue #17 — an optional Vim-style editing mode for the SQL editor.

• Ctrl+Shift+V toggles between Vim and standard editing mode
• Preference persisted to ~/.config/pgrsql/vim_mode
• Mode indicator shown in status bar (NORMAL/INSERT/VISUAL/V-LINE)

Normal Mode

Key	Action
h/j/k/l	Move left/down/up/right
w/b/e	Word forward/backward/end
0/$/^	Line start/end/first non-whitespace
gg/G	Go to first/last line
i/a/I/A	Enter Insert mode (at cursor/after/line start/line end)
o/O	New line below/above, enter Insert
v/V	Enter Visual/Visual-Line mode
x	Delete character
dd/dw/d$/d0	Delete line/word/to-end/to-start
yy/yw/y$	Yank line/word/to-end
cc/cw/c$	Change line/word/to-end
p/P	Paste after/before cursor
Count prefix (e.g., 3j)	Repeat motion N times

Insert Mode

• Standard editing keybindings
• Esc returns to Normal mode

Visual/V-Line Mode

• Motions extend selection
• d/y/c operate on selection

Changes

File	Changes
src/ui/app.rs	Added VimMode enum, vim state fields, handle_vim_normal_input(), handle_vim_insert_input(), handle_vim_visual_input(), vim helpers, toggle keybinding, preference persistence
src/ui/components.rs	Added vim mode indicator in status bar, updated help overlay

Test plan

• All 124 existing tests pass
• cargo clippy clean
• cargo fmt clean
• Manual test: Ctrl+Shift+V enables vim mode, status bar shows "NORMAL"
• Manual test: i enters insert mode, Esc returns to normal
• Manual test: h/j/k/l navigate, w/b word motions work
• Manual test: dd deletes line, yy yanks, p pastes
• Manual test: v enters visual mode, d deletes selection
• Manual test: Ctrl+Enter/F5 executes query in all modes
• Manual test: Ctrl+Shift+V disables vim mode

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