fix(dashboard): Top Senders — accurate counts and no stacked bars

[CafeLungo]

Two related fixes for the dashboard's Top 10 Senders widget.

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1. Counts were wildly inaccurate (backend)

Problem

On a ~200k-email archive the top bar maxed out around 25 — effectively a random/low
set of senders rather than the real leaders.

Root cause

SearchService.getTopSenders() reads Meilisearch's from facet distribution
(index.search('', { facets: ['from'] })). But configureEmailIndex() never set any
faceting config, so Meilisearch applied its defaults:

• maxValuesPerFacet: 100 — only 100 of the many thousands of distinct senders are returned
• sortFacetValuesBy: 'alpha' — those 100 are the alphabetically-first addresses, not the most frequent

getTopSenders() then sorted that arbitrary alphabetical slice by count and took the
top 10 — surfacing the top of an alphabetical subset, never the true high-volume
senders. The same defect affected getFacetValues() (facet typeahead suggestions).

Fix

Configure faceting on the emails index to sort facet values by count and raise the cap:

faceting: {
    maxValuesPerFacet: 1000,
    sortFacetValuesBy: { '*': 'count' },
},

Meilisearch now returns the highest-frequency facet values, so the widget sees the
real leaders. Settings-only change, applied at startup via configureEmailIndex() —
no reindex required.

Verification

Ran against a live ~200k-email index; Meilisearch's facet distribution now matches the
Postgres ground truth almost exactly (Amazon 8,338 vs 8,340, Facebook 7,178 vs 7,179, …).

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2. Two senders rendered as one stacked bar (frontend)

Problem

Two senders sometimes rendered as a single stacked bar (e.g. a "Facebook" row showing
1,925 + 4,832 in one bar) instead of two separate bars.

Root cause

TopSendersChart.svelte keys each bar on sender — the y-axis category — after
overwriting it with the resolved display name (senderName || address). Facet values
(addresses) are unique, but two distinct addresses can resolve to the same display
name (notification+…@facebookmail.com and update+…@facebookmail.com both → "Facebook").
Sharing a y value makes layerchart band them into one stacked bar.

Fix

Disambiguate the category: keep the bare name only when it's unique among the top
senders; when a name is shared, append the (unique) address so each sender keeps its
own bar. Bare unique names (Amazon.com, LinkedIn, …) are unchanged.