#296 r2: restore URL column to position 6 and widen to 340px

Reverts PR #297's column reorder (owner override): URL returns to position
6 in both list tables (Paste, Type, Size, Views, Created, URL, ID; /mine
without Views), data-sort attrs and td cells move back together. Keeps
#297's non-position improvements: /mine URL uses the shared .col-url class
instead of reusing col-e, and mine's ID column returns to .col-f (100px).

Widens .col-url 150px -> 340px so a ~20-char slug displays fully at
1400x900; genuinely long custom slugs keep text ellipsis. No mobile media
query needed: <=640px already scrolls the table horizontally (min-width
720px), verified at 375x812.

Verified via CDP probe at 1400x900 and 375x812 on /history and /mine:
column order, colgroup/header alignment, sort arrow right of label, no
other column clipped, no CSP changes (no inline styles added).

internal/web/static/app.css

@@ -896,7 +896,7 @@ button[type="submit"]:focus-visible,
 .col-f { width: 100px; } .col-g { width: 190px; }
 /* #255: history's URL column had its own narrow width (col-f doubles as
    /mine's ID column); give it a dedicated class. */
-.col-url { width: 150px; }
+.col-url { width: 340px; }
 /* #210: /mine rows render a delete button cell that had no declared column,
    so under table-layout:fixed it overlapped the ID column. */
 .col-del { width: 64px; }