Exercise detail page UX tweaks

[fiddur]

Summary

• Enable both HR and HRV by default on exercise detail chart
• Fix HRV Y-axis scale: start from 0, increase right margin (60→110px) to fit dual axes
• Move notes textarea to full-width layout below other edit fields
• Replace hardcoded "Back to Day View" link with contextual browser back navigation
• Add active calories display (summed from metric time series) on exercise detail

Test plan

• Open exercise detail, verify HR and HRV both toggled on by default
• Verify HRV axis starts from 0 and both axes render without clipping
• Edit an exercise's notes, verify textarea is full width
• Navigate to exercise detail from different pages, click Back, verify it returns to previous page
• Verify active calorie data shows for exercises that have it

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 0% with 35 lines in your changes missing coverage. Please review.
✅ Project coverage is 27.37%. Comparing base (4303272) to head (0692a19).
⚠️ Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
apps/web/src/pages/EntityDetail/ExerciseDetail.tsx	0.00%	19 Missing ⚠️
.../src/pages/EntityDetail/EditableActivityFields.tsx	0.00%	9 Missing ⚠️
apps/web/src/pages/EntityDetail/ActivityChart.tsx	0.00%	5 Missing ⚠️
apps/web/src/pages/EntityDetail/index.tsx	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #242      +/-   ##
===========================================
- Coverage    27.39%   27.37%   -0.03%     
===========================================
  Files          176      176              
  Lines        23642    23660      +18     
  Branches      2104     2104              
===========================================
  Hits          6476     6476              
- Misses       17127    17145      +18     
  Partials        39       39              

Flag	Coverage Δ
android	8.77% <ø> (ø)
backend	57.58% <ø> (ø)
web	3.02% <0.00%> (-0.01%)	⬇️

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