Align coding-standards docs + AppPrefs with the no-final-on-parameters convention

[almothafar]

Problem

The stated code-style convention is: final on local variables, but NOT on method/constructor parameters in new/edited code (legacy code keeps final params and migrates incrementally as it's touched). But the guideline docs still say the opposite, so the docs and the convention contradict each other:

• .claude/guidelines.md (AI-agent rulebook): "Immutability: Use final for all method parameters and local variables where possible" — the all method parameters clause is now wrong.
• CODE_REVIEW_GUIDELINES.md (human-facing): the "Use final for Variables Not Reassigned" section and several examples show final on parameters (e.g. public void process(final String input)).

Both files carry a "keep these two in sync" preamble, so they must be updated together.

This surfaced during the two-axis review of #166 / PR #199: AppPrefs.vibrateEnabled(final Context context) was flagged as a hard violation of the convention, while the docs technically endorsed it. That final has since been dropped in #199, which leaves AppPrefs mixed (the new method has no final param; the ~7 legacy methods still do).

Scope (one PR)

1. Docs — update both guideline files so they state the actual convention: final on locals, not on parameters; note that legacy code keeps final params and migrates incrementally when a method is otherwise edited. Fix the inline examples so they don't teach final params.
2. AppPrefs — drop final from the parameters of the remaining legacy methods (criticalLevel, warningLevel, batteryLevels, setBatteryLevels, drainLimitPph, clampDrainLimit, private prefs) so the file is internally consistent — a concrete reference application of the corrected convention. (final on locals stays.)

Not in scope: a codebase-wide final-param sweep. Legacy files migrate incrementally as they're touched, per the convention.

Acceptance criteria

• Both guideline docs state "final on locals, not on parameters" and stay in sync; no example shows a final parameter.
• AppPrefs has no final on parameters; final on locals unchanged.
• Build + unit tests + lint green (no behavior change — final on a parameter is a no-op modifier).