refactor: convention alignment — helpers, constants, imports, tests (#1950)

refactor: convention alignment — helpers, constants, imports, tests (#1950)

Convention alignment refactor across the monorepo — no behavior change.

Helpers adoption:
- adopt isEmpty() helper for emptiness checks (9 sites); OCPPServiceUtils.ts:1113 preserved with anchor comment (already-trimmed input; isEmpty would redundantly re-trim)
- adopt Constants.UNIT_DIVIDER_KILO for kW→W conversion (4 sites)
- use JSONStringify for unknown MCP tool payload (Map/Set-safe replacer)

Constants promotion:
- promote inline literals to named constants (MAX_ITEMS_PER_REPORT_MESSAGE with OCPP 2.0.1 §2.1.16/§2.1.18 spec-anchor comment; AVG_ENTRY_SIZE_BYTES; DEFAULT_EXPONENTIAL_BACKOFF_BASE_DELAY_MS; DEFAULT_AUTH_CACHE_TTL_SECONDS; DEFAULT_PERSIST_STATE)
- coverage gap: isNotEmptyArray adoption at ConfigurationValidation.ts:124

Imports discipline:
- add .js extension to relative import in ui-web/vitest.config (ESM/NodeNext)
- use component barrels instead of deep imports in tests (10 sites)

Test literals:
- use DEFAULT_HOST/DEFAULT_PORT constants in ui-common tests (72 sites: 32 in config.test.ts + 40 in WebSocketClient.test.ts)

Documentation anchors:
- document Number.parseInt/parseFloat convention exceptions (3 comments covering 4 call sites where convertToInt/convertToFloat cannot substitute)

Quality gates: typecheck clean, lint clean, 2954/2960 tests pass, 0 fail.

Diff: +140/-113 across 28 files.