Fix point tag overlap in day view

[fiddur]

Summary

• Point tag text overflow: Left-align diamond markers within their lane so text labels stay bounded within lane width instead of overflowing into neighboring lanes
• Lane assignment loss: Preserve original lane assignments in mergeSmallItems for non-merged items — previously all items were reset to lane: 0 when any tiny item triggered merging, causing overlapping items to render on top of each other

Test plan

• Open day view with two point tags at the same time → they should display side by side in separate lanes
• Open day view with a span tag and overlapping point tag → point tag text should not overflow into the span tag's lane
• Verify merged cluster behavior still works for many tiny items close together

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 0% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 27.26%. Comparing base (5a39340) to head (f6524b0).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
apps/web/src/pages/DayView/index.tsx	0.00%	28 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #246   +/-   ##
========================================
  Coverage    27.26%   27.26%           
========================================
  Files          176      176           
  Lines        23752    23751    -1     
  Branches      2122     2122           
========================================
  Hits          6476     6476           
+ Misses       17237    17236    -1     
  Partials        39       39           

Flag	Coverage Δ
android	8.57% <ø> (ø)
backend	57.58% <ø> (ø)
web	3.02% <0.00%> (+<0.01%)	⬆️

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