docs(readme): new screenshots and GIFs that follow the reader's theme, and no em dashes

The README opens with what TreeMap 5.2.0 looks like: the Treemap view of a demo home folder, the
scan and drill-in GIFs, and the Dashboard, Disk City, Duplicates and the Clean Up cart. Each still
comes in a light and a dark version (docs/media, WebP at 1600 px, about 2 MB for all eighteen), and
GitHub shows the one that matches the reader's theme. The shots are from a synthetic demo home
folder; no real folder was scanned for them (docs/media holds them; nothing there is packaged into
the installers).

Every em dash is gone: a pair inside one sentence became commas or brackets, a lone one a comma
or a colon. The macOS first-launch heading keeps its old link target, so the issue template's and
the install note's links still land on it. docsTruth pins the Held-up space sentence word for word;
its pattern now reads the brackets and commas that replaced that sentence's dashes, and says the
same thing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: Prithvi-Web