The tab-group number counts all tabs, not the tabs in one folder

[astetic-dev]

What happens

Settings -> "Group tabs from the same folder from [N] tabs" is set to 4. With nine
sessions open, two of which live in the same folder, those two already collapse into one
group tab. A folder is bundled at two tabs, not at four.

Why

tabRows() (src/main.js) reads the number as a threshold on the total number of
tabs, and then bundles every folder that has more than one session:

if (!settings.tabGroups || alle.length <= settings.tabGroupAt) {
  return alle.map((s) => ({ groep: false, leden: [s] }));   // nothing grouped
}
...
rijen.push({ groep: leden.length > 1, leden, key: k });      // group size hard-coded at 2

So the number decides whether grouping happens at all, while the group size is fixed at
two. That is what #90 designed ("Below the threshold (default 10 tabs, a setting) nothing
changes"), but it is not what the label promises and not what someone typing "4" expects.

Second, smaller mismatch: "from 4 tabs" never fires at 4. The comparison is
alle.length <= tabGroupAt, so grouping starts at 5.

Behaviour we want

The number is the group size, per folder: a folder collapses into one tab as soon as
it holds that many sessions. 0 stays "never bundle"; the smallest real size is 2.

The total-tab gate goes away. It bought little: grouping only ever merges sessions that
share a folder, so the "ten agents in ten different folders" crowding that the gate was
meant to catch was never helped by it anyway.

Changes

• tabGroupAt (all tabs) becomes tabGroupSize (tabs in one folder), default 3.
• Folders below the size keep their own tabs, each in its own place in the bar.
• A stored number other than the old default 10 carries over unchanged -- whoever typed a
  number meant the folder, because that is what the label says. Someone still on 10 gets
  the new default instead of a silently dead setting.
• Label and help text in nl + en say "tabs in that folder", and 0 = never.

The NEXUS Agent Launcher fork runs the same src/main.js and has the same bug; it gets
the same patch.