Add thematic categories with icons to the shortcuts window

[niksedk]

Addresses #12316.

Design prototype that this implements: category tile filter + icon column + "Active in" context column.

What changed

• New thematic grouping (ShortcutGroup): General, File, Video, Waveform, Subtitle list view, Text box, Grid + text box, Synchronization, Translate, Search, Tools, AI. It is display-only — the existing ShortcutCategory still drives dispatch/hashing and is untouched.
• "Active in" column — the old Category column, renamed. Shows the focus context where the shortcut fires ("Everywhere" for the old General).
• Icon column — each row gets a small colored group icon (mdi font icons, tinted per group) with the group name as tooltip, so long lists stay scannable while scrolling (the effect the emoji-prefix experiment in the issue was reaching for, without touching shortcut names or translations).
• Group tile strip — a wrap of tiles under the search box (icon + name + count, plus an "All" tile) that filters the grid per group.
• 145 shortcuts re-tagged out of the General bucket (previously 321 of ~420 rows said "General"): 64 Video, 26 File, 24 Tools, 9 Search, 8 Translate, 7 Sync, 7 AI. Tags are a single extra argument on AddShortcut, so adjusting a shortcut's group later is a one-word change.
• New language strings (CategoryFile, CategoryVideo, …, ActiveIn, ActiveInEverywhere); other languages fall back to the English defaults until translated.
• Window default size bumped 760×650 → 900×720 for the two extra columns.

Notes for review

• Group colors are fixed mid-saturation tones chosen to read on both dark and light themes (ShortcutGroupUi.GetBrush).
• Icon names were verified to exist in the bundled MaterialDesign icon font.
• Builds clean; needs a manual UI pass (Options → Shortcuts) since I did not launch the GUI.

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