chore: lint imports that are not at the top-level of a file

[SuMayaBee]

Description

Turns on ruff's PLC0415 (import should be at the top level of a file), so a lazy import gets caught by lint instead of only in review. Bumps the pinned ruff pre-commit rev to v0.15.5, the rule only became stable there.

Every import this flagged was already deliberately lazy, so each gets a # noqa: PLC0415 and a comment naming its measured cost.

Fixes #68

AI Disclosure

• Tool & Model: Claude Code

• I have tested all AI-generated content in my PR.

• I take responsibility for all AI-generated content in my PR.

Checklist

• Tests added and are passing

[SuMayaBee]

@ahuang11 It's ready for review, here is the detail overview:

Import	Cost	Why it stays nested
panel (app.py)	~704 ms	only pls serve needs it
panel_live_server.pages (app.py)	~1510 ms	only pls serve needs it
panel_live_server.app (cli.py)	~532 ms	only pls serve needs it
panel_live_server.server (cli.py)	~528 ms	only pls mcp needs it
panel / bokeh (utils.py)	~704 ms / ~289 ms	cli.py imports utils to fix the Windows DLL path before any heavy import
playwright (screenshot.py)	~31 ms	kept for robustness, not cost: a broken install should disable screenshots, not pls mcp