Rebrand LineWatch to UplinkWitness without breaking runtime compatibility

[LucaXTech]

Summary

This PR completes the public rename from LineWatch to UplinkWitness while deliberately preserving runtime identifiers that existing installations depend on.

Public-facing changes

• repository/README branding → UplinkWitness
• dashboard page title/header → UplinkWitness
• ISP report headers → UplinkWitness
• monitor log prefix and HTTP User-Agent → UplinkWitness
• installer/configuration messages and systemd descriptions → UplinkWitness
• contributor/security/roadmap/compatibility/testing docs → UplinkWitness
• bug-report template → UplinkWitness
• adds docs/RENAMING.md with upgrade guidance
• adds a dashboard branding regression test

Compatibility intentionally preserved

This PR does not rename:

• linewatch.service
• linewatch-dashboard.service
• LINEWATCH_* environment variables
• data/linewatch.sqlite3
• existing event/database schemas
• production installation directories

That keeps v1.0/v1.1 upgrades and historical data intact. Existing production installs should not rename their local directory merely to match the public brand.

Release history

Releases through v1.1.0 remain historically identified as LineWatch. Subsequent releases use the UplinkWitness public identity.

Validation

• runtime logic/classification intentionally unchanged
• shell service identifiers unchanged
• new test checks dashboard title/header branding
• CI should run the existing Python 3.11/3.13 suite, compile checks and shell syntax validation

Closes the implementation portion of #6; the issue can be closed after merge and post-merge verification.