feat(themes): add vesper theme

[mihaicrisann]

Adds Vesper as a built-in dark theme, ported from the palette in raunofreiberg/vesper.

• background #101010, surface #1C1C1C, panel #232323 map to vesper's editor / input / active-selection backgrounds
• primary/accent #FFC799 is vesper's focus border and button color, error #FF8080 its error foreground, success #99FFE4 its string token
• registered in SQLIT_THEMES and listed under DARK_THEMES in the picker

pytest tests/ -k sqlite, pytest tests/cli/ and pytest tests/ui/ all pass locally (51 + 10 + 407).

[Maxteabag]

Captured Vesper at commit 8766c1992146059d5492e197bec36327ed030c4e in six UI states. Expand each screenshot below.

• Disconnected explorer / empty workspace
• Results focused, with a selected cell and alternating rows
• Query editor in INSERT mode with SQL syntax highlighting
• Theme picker with Vesper selected
• Edit connection dialog with a focused input
• Query error dialog after querying a nonexistent table

Captured from the running app with Textual Pilot at 120 × 40 terminal cells, exported as SVG and rendered to PNG with Chromium. Textual 6.11.0; true color enabled; isolated configuration and synthetic data in a temporary SQLite database. The successful query returned five rows; the error screenshot comes from executing SELECT * FROM missing_orders;.

The peach focus/selection accents, mint INSERT indicator and result cursor, and red error styling are visible. SQL syntax highlighting retains the editor's existing multicolor palette. These are UI captures, not a full test-suite run.

Disconnected

Results focused

Editor insert mode

Theme picker

Connection dialog

Query error

[Maxteabag]

@mihaicrisan04 Looks really nice! Thanks for the PR