fix: dimension table freezes when resizing the dimension column in DimensionTable.svelte

[nishantmonu51]

• Resizing the dimension column in an expanded dimension table froze the browser tab for ~45s until Svelte threw effect_update_depth_exceeded.
• Broken by refac: unified expression filter component across explore, canvas, alerts and reports #9746, which migrated DimensionTable.svelte to runes and converted the column width $: block to an $effect verbatim. The effect assigned estimateColumnSize and then read it back through estimateColumnSize[0] = manualDimensionColumnWidth. In Svelte 5 an effect that reads a source it just wrote registers it as its own dependency and reschedules itself, so the first manual resize looped forever. That branch only runs after a resize, which is why the table loaded fine.
• Fix: compute estimateColumnSize with $derived.by instead of writing $state inside an effect.
• No other table is affected: the measure column headers never wire up a resize handler, and the pivot, TDD and alert preview tables are still on Svelte 4 syntax where this mechanism does not apply.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!