fix: resolve all lint warnings and promote warn rules to error

[fiddur]

Summary

• Fix all ESLint warnings across backend and frontend, then promote the five warn-level rules (complexity, max-depth, max-lines, no-shadow, object-shorthand) to error in eslint.config.js so new violations fail CI.

Backend changes

• oura-sync.ts: Extract data processing functions into new oura-process.ts to fix max-lines (540→~50 lines remaining)
• mutations.ts: Extract notes, restore, and custom-metrics services into dedicated files to fix max-lines (596→~200 lines)
• queries.ts: Extract enrichActivity() and enrichSleepFields() helpers to fix complexity (16→under 15)
• mutations.ts addMetric: Extract validateCustomMetricRange() helper to fix complexity (16→under 15)

Frontend changes

• Header.tsx: Data-driven NAV_LINKS array with NavLink component (complexity 18→under 15)
• DayView: Extract drawPointMarker() and drawBlockOverlay() helpers (complexity 23→under 15)
• DataSources: Extract shared SaveStatusIndicator, StatusBanner, DataTypesList, LoginRequired into shared.tsx; refactor CalendarsSource, LastFmSource, RescueTimeSource
• EntityDetail: Split monolithic EntityContent into ActivityContent, TagContent, ProductivityContent, MetricContent; extract MetricDetail.tsx, ProductivityDetail.tsx, TagDetail.tsx
• ExerciseDetail: Extract ExerciseStats component (complexity 20→under 15)
• TagMappingsSettings: Extract RowStatusIndicator, IconPreview components (complexity 21→under 15)
• LastFmTagRulesSettings: Extract AddRuleForm into separate file, RuleEditForm component, buildUpdateBody/getArtistUpdates helpers (complexity 18,17→under 15; max-lines 671→443+262)

ESLint config

All five rules promoted from warn to error:

complexity: ['error', 15]
'max-depth': ['error', 4]
'max-lines': ['error', { max: 500, skipBlankLines: true, skipComments: true }]
'no-shadow': 'error'
'object-shorthand': 'error'

Verification

• pnpm fix — clean (zero warnings/errors)
• pnpm check — clean (tsc + eslint pass)
• Backend tests: 972/972 pass across 53 test files

[codecov]

Codecov Report

❌ Patch coverage is 84.57249% with 83 lines in your changes missing coverage. Please review.
✅ Project coverage is 32.43%. Comparing base (a2c37e5) to head (7e5dee5).
⚠️ Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
apps/backend/src/services/notes.ts	2.00%	49 Missing ⚠️
apps/backend/src/services/restore.ts	4.76%	20 Missing ⚠️
apps/backend/src/services/queries.ts	67.44%	14 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #287      +/-   ##
===========================================
+ Coverage    31.88%   32.43%   +0.54%     
===========================================
  Files          181      185       +4     
  Lines        22992    22651     -341     
  Branches      1886     1889       +3     
===========================================
+ Hits          7331     7346      +15     
+ Misses       15627    15271     -356     
  Partials        34       34              

Flag	Coverage Δ
backend	58.97% <84.57%> (+0.03%)	⬆️

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