dotbot: UX polish - default dashboard, --headless, clearer CLI errors

[geonnave]

A grab-bag of CLI and docs UX fixes found while testing the dotbot workflow
end to end. Low-risk: mostly docs, plus one behavior change on the controller.

Dashboard on by default

dotbot run controller (and run simulator) now open the web dashboard
automatically; pass --headless to suppress it (it's still served). The old
webbrowser setting is renamed to headless everywhere - the flag, the
ControllerSettings field, the [run.controller] config key, and the docs -
so the concept has a single name. The qr demo keeps its own opt-in
--webbrowser (a phone scans the QR there, so no-browser is the right default).

Clearer wording and errors

• "DotBot" instead of "bot"/"robot" in help text and docs; "SEGGER Embedded
  Studio" spelled out instead of "SES".
• pip install pydotbot instead of pip install --pre pydotbot (no longer a
  pre-release).
• "no config file found" now names where it looked (./dotbot.toml and
  ~/.dotbot/config.toml).
• The missing-swarm-id flash error spells out the hex format and warns that
  -s / --sn-starting-digits is the J-Link serial prefix, not the swarm id.
  This is the exact confusion in Documentation #272 (flash-mari-gateway -s 10 read as a
  swarm id).

README

The calibration section leads with pip install 'pydotbot[calibrate]' and how
to find a Device Addr from dotbot swarm status.

Closes #272.

Validated: full test suite (403) green, pre-commit clean. The controller
dashboard/headless behavior is worth a quick bench check.

[codecov]

Codecov Report

❌ Patch coverage is 91.36691% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.50%. Comparing base (a26bc89) to head (bbcb28b).
⚠️ Report is 92 commits behind head on main.

Files with missing lines	Patch %	Lines
dotbot/cli/swarm.py	0.00%	9 Missing ⚠️
dotbot/cli/device.py	84.61%	2 Missing ⚠️
dotbot/cli/swarm_lh2.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #275      +/-   ##
==========================================
+ Coverage   82.55%   83.50%   +0.94%     
==========================================
  Files         107      121      +14     
  Lines        9964    11233    +1269     
  Branches      570      570              
==========================================
+ Hits         8226     9380    +1154     
- Misses       1735     1850     +115     
  Partials        3        3              

Files with missing lines	Coverage Δ
dotbot/cli/_fw_helpers.py	95.79% <ø> (-0.73%)	⬇️
dotbot/cli/_swarm_flash.py	100.00% <100.00%> (ø)
dotbot/cli/fw.py	90.60% <ø> (+3.00%)	⬆️
dotbot/cli/joystick.py	100.00% <ø> (ø)
dotbot/cli/keyboard.py	100.00% <ø> (ø)
dotbot/cli/main.py	100.00% <100.00%> (ø)
dotbot/config.py	98.02% <100.00%> (+2.13%)	⬆️
dotbot/controller.py	83.14% <100.00%> (+0.57%)	⬆️
dotbot/controller_app.py	96.90% <ø> (+0.13%)	⬆️
dotbot/tests/test_calibration_lighthouse2.py	100.00% <100.00%> (ø)
... and 6 more

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